You might have noticed for xrdp on Debian (but quite possibly with a lot of other Linux tools and other Linux distributions) the user limits (described in /etc/security/limits.conf) are not enforced. Which meant in my case that any session open with xrdp was opened with a max number of open files (nofile) set to 1024.

To fix this, edit the file /etc/pam.d/common-session and add the following line:

session    required